Garnet compositions



May 13, 1969 Filed Sept. 30, 1964 FIG.

FIG. 2

s. GELLER ET AL 3,444,084

GARNET COMPOSITIONS I Sheet I of 3 B-GAUSS H- 2 3 OERSTEDS FIG. 3

B-GAUSS -BOO B-GAUSS s. GELLER 'NVENTORS E. A. NESB/TT A T ORNEV y 13,1969' s. (SELLER ETAL 4 GARNET COMPOSITIONS Filed Sept. 30, 1964 Sheet 3or a F/G 5 a a4 uss H. E 6 4 2 o 2 4 e omsrws H- a 6 5 i s' omsrms8-GAUS5 F/G.7 600 a A OERSTEDS H... 5 6 OERSTEDS a s i 0 2 2: 6 omsrws y1969 s. (SELLER ETAL GARNET COMPOSITIONS Sheet Filed Sept. 30, 1964 FIG.

DETECTION C/RCU/TFPV 3,444,084 GARNET COMPOSITIONS Seymour Geller,Camarillo, Calif., and Ethan A. Nesbitt,

Berkeley Heights, N.J., assignors to Bell Telephone Laboratories,Incorporated, New York, N.Y., a corporation of New York Filed Sept. 30,1964, Ser. No. 400,508 Int. Cl. C04b 35/40 US. Cl. 252-6257 3 ClaimsABSTRACT OF THE DISCLOSURE 'Ihe hysteresis loop squareness ratio ofyttrium, iron, garnet (YIG) and related compositions containing limitedamounts of gallium and aluminum is improved by partial substitution ofeuropium or terbium for yttrium. Both single crystal and polycrystallinematerials are included.

This invention relates to garnet compositions showing enhancedhysteresis loop characteristics and to devices utilizing suchcompositions.

It is unnecessary to discuss at any length the role played by thehysteresis loop in modern \day technology. The remanence of the loop isthe memory that serves in all magnetic switching elements from thesimple, single element core to the mammoth computer which may usemillions of such elements.

While a1 lof the loop characteristics of a material are of concern inthe fabrication of any particular device, certain of the characteristicsare invariably of importance. Any memory device is dependent onremanence, that is, the magnetization retained by the magnetic elementin the absence of an applied field. The ratio of this quantity to thematerials saturation, B /B the squareness ratio, is a measure of thepercentage of the applied field retained upon its removal. It is clearthat this is of the nature of an efficiency term, so that a low value ofsquareness ratio is indicative of wasted energy.

It is well known that squareness ratio is of importance in other designconsiderations. For example, many composite memory devices operate onthe principle of coincident current write. Such elements are operated byuse of two associated current windings, which in the write function mayeach carry currents of a magnitude of the order of half that required toproduce a field necessary to magnetically saturate the element. Suchelements are switched only when both associated windings are carryingtheir designated half currents. The more skewed the hysteresis loop,that is, the lower the value of the squareness ratio, the greater thepossibility of a half current producing partial switching. By reason ofthis condition, a leakage current may, when reinforcing a half current,be adequate to produce switching. Leakage currents increase for closerspacing of elements. It therefore follows that improved squareness ratiopermits closer packing of memory elements.

Since a high value of squareness ratio indicates a welldefined easydirection of magnetization which, in turn, signifies a low resistance todomain alignment in such direction, switching speed is expected to be afunction of squareness. This is now accepted by workers skilled in theart, and those magnetic memory elements capable of the faster responsetimes which have been reported depend in part for this attribute on thehigh squareness ratios of the hysteresis loops of the magnetic materialsutilized.

Many memory devices utilize magnetic garnet elements based on thestoichiometry Y Fe O and sometimes designated yttrium-iron-garnet, orYIG. Such compositions, as well as materials based on such compositionsand dif- 3,444,084 Patented May 13, 1969 ice In accordance with thisinvention, it has been discovered that partial substitutions of europiumand/or terbium for yttrium in YIG, as well as in partially aluminumorgallium-substituted YIG, results in improved hysteresis loop squareness.The compositional variations so defined, while perhaps of primetechnological significance at this time as applied to ceramic bodies,are usefully applied to single crystals of the same compositions. Againfrom the technological standpoint, prime significance appears to centeron the lower saturation materials produced by partial substitution ofaluminum or gallium, and such materials constitute a preferredembodiment of this invention. Increasing the hysteresis loop squarenessresults in the feasible or improved operation of a broad class ofmagnetic memory elements. Elements utilizing the compositions set forthherein constitute an aspect of the invention.

The materials of this invention may be represented as having acomposition within the range defined by:

where R is europium and/or terbium, M is aluminum and/or gallium, xequals from 0.3 to 2.0, and y equals from 0 to 2.0. The choice ofeuropium and terbium in this composition is extremely critical. In fact,no other rare earth which may be substituted for yttrium has been foundto share the advantage of either of these elements.

The use of aluminum and gallium up to the maximum content indicated is,of course, based on the prior art. These 'are the reduced saturationmaterials rapidly gaining acceptance for device use. Since suchcompositions constitute a preferred embodiment of this invention forcertain device uses, and since a significant reduction in saturation isachieved only by use of at least 0.1 atom of aluminum or gallium performula unit, such a pre ferred minimum value of y for these purposes isso indicated. The values of x, that is, the limit on europium and/orterbium content, are based largely on experimental work, the results ofmuch of which are reproduced here in graphic form. While lessersubstitutions of terbium or europium do produce an improvement insquareness, significant improvement for many device applications isrealized only upon incorporation of a minimum amount of 0.3 in theformula. Replacement of two-thirds of the yttrium, that is, use of twoatoms of either of these rare earths, still results in significantimprovement in squareness ratio. However, since some falloif inimprovement is perceived, the limit of two as the maximum value of x isconsidered expedient. Variations in the value of y, that is, on aluminumand/or gallium content, are, of course, based on the prior art. It hasbeen observed generally that incorporation of greater than the maximumindicated of either of these metals results in a need for a significantincrease in firing temperature in the preparation of the ceramiccomposition and also results in a concomitant decrease in squarenessratio generally to levels unacceptable for many memory applications.Nevertheless, partial substitution of europium and/or terbium foryttrium results in improvement in squareness values for compositionscontaining greater amounts of aluminum or gallium. For the reasonsdiscussed in this paragraph, the composition set forth in the formulaabove is considered to define only a preferred compositional range inaccordance with this invention.

Description of the invention is expedited by reference to the drawing,in which:

FIG. 1 is a BH plot in units of gauss and oersteds, respectively, for asample of unsubstituted Y3Fe5012;

FIG. 2 is a similar plot for a garnet composition containing a smallamount of terbium;

FIG. 3 is a similar plot for a europium-substituted garnet;

FIG. 4 is such a plot for a sample of YIG containing a larger amount ofeuropium;

FIG. 5 is such a plot for a europium-substituted yttrium-iron-aluminumgarnet composition;

FIG. 6 is such a plot for a composition similar to that of FIG. 5containing a greater amount of europium;

FIG. 7 is such a plot for a composition similar to that of FIG. 6containing a still greater amount of europium;

FIG. 8 is such a plot for a composition similar to that of FIG. 5containing still more europium than the sample of FIG. 7;

FIG. 9 is a similar plot for an yttrium-iron-aluminum garnet compositioncontaining a still greater amount of europium;

FIG. 10 is a schematic representation of a memory array using elementsconstructed of the materials herein; and

FIG. 11 is a perspective view, partly in section, of another design ofmemory device utilizing a material of this invention.

Materials of this invention may be prepared by any of the severaltechniques customarily employed either in the ceramic or single crystalfield. One method found to produce satisfactory solid solutions(ceramics) involves the preparation of the solid solution directly. Inaccordance with this method, the necessary chemical elements, forexample, in the form of oxides are mixed in stoichiometric proportions.The mixture is then pressed into a pellet and prefixed at a temperaturein the preferred range of 1200 C. to 1300 C. for a period ofapproximately one-half hour. The pellet is then cooled, ground, andremixed, and is then pressed and refired at a temperature of from about1300 C. to about 1350 C. for a period of approximately One to threehours. Firing, possibly preceded by grinding, may be repeated one ormore times to obtain a homogeneous garnet structure or increased bulkdensity in accordance with the usually practiced procedure. The durationof the firing steps may be increased in accordance with conventionalpractice.

Single crystals of these compositions may be prepared by spontaneousnucleation from lead-oxide fluxes or from modified lead-oxide fluxes inaccordance with the procedures of I. W. Nielsen US. 'Patents No.2,957,827 and No. 3,050,047, or by use of the J. P. 'Remeika flux inaccordance with his process of US. Patent No. 3,079,240. Alternativeflux growth procedures, as well as seeded and fusion techniques, areknown, and, as would be expected, materials prepared by any of theseprocedures show improved properties.

For ease of comparison, characteristics plotted on FIGS. 1 through 9 aretaken from compositions all pre pared in accordance with a ceramicforming technique including firing schedules that are similar for allsuch samples. All of these samples have been examined by X-raytechniques and found to be homogeneous single phase. It is convenient toconsider this series of figures in two subgroupings. Group 1, FIGS. 1through 5, is based on garnet compositions containing no substitutionsfor iron. Compositions for this series may, therefore, be represented asR Y Fe O Terbium is found to be equivalent to europium in producingimproved squareness, and, one of the samples, that plotted on FIG. 2, isof a composition in which yttrium is partially replaced by that element.The compositions of FIGS. 3 and 4 are based on europium substitution.The results are conveniently tabulated below.

TABLE 1 x Squareness It has been noted that reduction of saturation ofgarnet materials by replacement of nonmagnetic elements, aluminum orgallium, for iron generally results in a sharp decrease in squarenessratio. FIGS. 5 through 9 demonstrate the efficiency of this inventionfor the retention of usable squareness ratios in such materials. Againfor comparison purposes, all samples dipected are of closely relatedcompositions. Dilution of moment is accomplished by replacement ofone-half atom of iron by aluminum in the formula unit. For the runsdepicted, the same rare earth, europium, in increasing amount from FIGS.5 through 9, is utilized. The results are summarized in the followingtable:

TABLE 2 x Squareness Firing conditions are again similar for the samplesrepresented by FIGS. 5 through 9. Deviation from any of these conditionsmay increase or decrease saturation and may result in a variation insquareness. Use of terbium rather than europium, while similar ineffect, may not be identical in magnitude. Experimental results of whichthe plotted data are illustrative have established an improvement insquareness for other conditions and other compositions for europium orterbium substitution on a 1:1 comparison. That is, comparison of twosamples, one containing three atoms of yttrium per formula unit and theother containing europium or terbium in any amount within the specifiedrange, shows an improvement in squareness ratio where the samples areotherwise identical, that is, same preparation conditions, same degreeand type of substitution for iron.

FIG. 10 depicts a bit-organized memory array disclosed and claimed inUS. Patent 2,825,891, and a more complete operation of this device maybe found there. In such a device, cores 1 through 9, all constructed ofa garnet composition herein, are interconnected through input windings ato f, and an output winding g, each connected to energizing means, notshown, which windings are single conductors. Each of the cores 1 through9 is initially in a magnetically rcmanent condition, with the directionof polarization signifying 0. The writing of a 1, characterized by aremanent condition with an opposite polarization in a given core, iseffected by supplying to each conductor associated with said core acurrent pulse having a value suflicient to produce a flux equal to orslightly more than one-half that required to saturate the core. In thecore 8, for example, a 1 is written by supplying a pulse of suchmagnitude through each of the conductors e through c, the cores 2, 5, 7,and 9 then being energized only by half currents. This pulse, producedin these four cores, is therefore insufl'icient to effect a change indirection of polarization. The readingout is effected similarly asdescribed, again using coincident half currents, however of negativedirection. This hase the effect of switching core 9 to its initialcondition, during the course of which a current pulse is induced inread-out winding g.

FIG. 11 depicts a word-organized memory array known as the wafiie iron.This array is constructed of a high magnetic permeability base plate 11being formed in such fashion as to include protruding posts 12 resembling those of a waffle iron in outward appearance, with a closelyfitting overlay sheet of magnetic material 18 of a garnet composition inaccordance with this invention positioned across the top of posts 12.The magnetic sheet 18 is advantageously clamped tightly against posts 12in order to minimize total reluctance of magnetic paths which includeboth the posts 12 and sheet 18. This clamping is here achieved by meansof a metallic pressure plate 19 above the sheet 18 and clamping screws20 between plate 19 and end portions of the base plate 11. Twoelectrical conductive paths, any one of windings 13 through 13 togetherwith any one of 15 through 15 each connected to energizing means, notshown, are associated with each of posts 12. Writing is accomplished incoincident fashion, supplying half currents in the manner described to aselected pair of such windings. Reading is accomplished by applicationof a subsequent interrogating signal to a word conductor 13, through 13so inducing an output signal in the particular conductor 15 through 15the polarity of which is indicative of the stored binary value. Acomplete description of the operation of such device is set forth in thereference noted above.

Many other devices beneficially incorporating compositions of thisinvention are known. They include the Laddic or U.S. Patent No.3,040,305, as well as various devices described in U.S. Patent No.2,736,880. Such devices may utilize open as well as closed flux pathswhich may have associated with them printed as well as wire or evenwaveguide conductive paths and may effect partial switching by passageof the current through the magnetic material itself. The suitability ofthe materials of this invention to all such devices is understood bypersons skilled in the art, in consequence of which the descriptionabove is considered exemplary only and in no way limiting.

The compositions of this invention have been discussed in terms of thegarnet formula. It is known that various additional modifications may bemade in such composition to bring about certain desired results. Forexample,

inert materials such as plastic fillers may be incorporated. Further, itis known that commercially suitable starting ingredients may containimpurities such as to result in as much as one percent by weight of suchingredients in the final composition. Where in the appended claimsreference is made to composition in terms of the garnet formula, it isto be understood that all such variations not considered essential tothe magnetic characteristic here under consideration, i.e., squarenessratio, are to be considered within the scope of the claims.

What is claimed is:

1. Magnetic material having high values of squareness ratio consistingessentially of the composition which may be represented as:

in which R is at least one rare earth selected from the group consistingof europium and terbium, x is a value of from 0.3 to 2.0, M is a metalselected from the group consisting of aluminum and gallium, and y is avalue of from 0.1 to 2.0.

2. Material of claim 1 in which M is aluminum.

3. Material of claim 1 which is polycrystalline.

References Cited UNITED STATES PATENTS 2,825,891 3/1958 Duinker 340-1742,938,183 5/1960 Dillon 25262.57 2,957,827 10/1960 Nielsen 25262.573,003,966 10/1961 Van Uitert 25262.57 3,040,305 6/1962 Gianola.

OTHER REFERENCES Belov et al.: Chemical Abstracts, vol. 56, June 1962,p. l3669a.

Villers et al.: Comptes Rendus, T. 255, No. 7, Aug. 13, 1962, pp.1196-8.

TOBIAS E. LEVOW, Primary Examiner.

I. COOPER, Assistant Examiner.

U.S. Cl. X.R. 340-174

